A communication system is formed, at a minimum, of a sending station and a receiving station interconnected by way of a communication channel. Information generated at the sending station is communicated to the receiving station upon the communication channel. A wide variety of different types of communication systems have been developed and are regularly utilized to effectuate communication of information between sending and receiving stations.
Advancements in communication technologies have permitted the development and construction of new types of communication systems. A radio communication system is exemplary of a type of communication system which has benefited from advancements in communication technologies. A communication channel formed between the sending and receiving stations of a radio communication system is defined upon a radio link. Because a radio link is utilized to form the communication channel, the conventional need of wireline connections between the sending and receiving stations is obviated. Thus, increased mobility is inherent in a radio communication system in contrast to a conventional wireline system.
The communication capacity of a radio communication system, however, is sometimes constricted as a result of bandwidth limitations. Only a limited amount of the electromagnetic spectrum is typically allocated to be used by a particular radio communication system and upon which communication channels can be defined. Communication capacity increase of a radio communication system is, therefore, sometimes limited by such allocation of bandwidth. Increase of the communication capacity of the radio communication system, therefore, is sometimes only possible by increasing the efficiency by which the allocated spectrum is used.
Digital communication techniques are among the advancements in communication technologies which have advantageously been implemented in various communication systems. Digital communication techniques permit the bandwidth efficiency of communications upon a communication channel in a communication system to be increased. Due to the particular need to efficiently utilize the bandwidth allocated in a radio communication system, the use of such digital techniques is particularly advantageously utilized in a radio communication system.
A key operation in digital communication is converting the analog signal received by the receiving station at the receiving antenna to a digital representation of the signal. Such a process typically involves an analog amplifier which increases the amplitude of the analog signal received at the antenna and an analog-to-digital converter (ADC) circuit which transforms the amplified signal to a digital representation of the signal. The resulting output of the ADC is a digital value presented over a data bus representing the analog input signal. The receiving station can then digitally process the digital signal from the ADC.
Generally, the ADC provides a digital signal corresponding to the analog input signal which ranges in value from a minimum value to a maximum value. If the analog input signal increases beyond the ADC's maximum output value, the digital signal remains at the maximum value even though the analog signal is greater than the corresponding digital value. Such a situation is often referred to as “clipping” because the top of the analog signal is not registered by the ADC.
Clipping is highly undesirable in digital communication because it results in lost information. One way to avoid clipping is to control the amplification level of the analog signal at the amplifier before it is input to the ADC. If the signal begins to slowly increase close to the maximum level of the ADC the amplification level is reduced, causing the analog signal to decrease to a safer level. To further insure against clipping, the analog signal is amplified to value smaller than the maximum value of the ADC, leaving a safety margin between the input signal level and the maximum level known as “headroom”. Providing a sufficient headroom level ensures that clipping does not occur even when the analog signal quickly jumps in amplitude faster than the receiver can adjust the amplification level of the amplifier.
Although providing headroom between the analog signal and the maximum value of the ADC helps prevent clipping, it also decreases the resolution of the digital signal because less than the full-scale of the ADC is utilized to convert the analog signal to a digital signal. Resolution loss is especially costly when the analog signal includes a high level of interference greater in amplitude than the desired signal. Generally, if the interference is greater than the desired signal, the amplification level and headroom is dictated by the interference signal and not the desired signal. This, in turn, causes the desired signal to receive an even smaller fraction of the ADC's full-scale reading at a lowered digital resolution.